The September 11 attacks also precipitated a focus on domestic security issues and the creation of a new cabinet-level federal agency, the Department of Homeland Security. The USA PATRIOT Act of 2001 was passed soon after the attacks, giving law enforcement agencies sweeping search and surveillance powers over US citizens without a warrant. This led to the creation in 2002 of the Information Awareness Office (IAO), led by John Poindexter. The IAO has initiated a program called Total Information Awareness, amended in May 2003 to Terrorist Information Awareness (TIA), with the aim of developing technology that would enable it to collect and process massive amounts of information about every individual in the United States, and trace patterns of behavior that could help predict terrorist activities. The information the IAO would gather includes Internet activity, credit card purchase histories, airline ticket purchases, car rentals, medical records, educational transcripts, driver's licenses, utility bills, tax returns, and other available data. Critics of the IAO believe it goes too far in the sacrifice of civil liberties and privacy, putting in place an Orwellian infrastructure prone to abuse. Many major events the United States has hosted since September 11, 2001 have been designated National Special Security Events (NSSE), because of concerns of terrorism. Metropolitan Police Department of the District of ColumbiaChiefCharles Ramsey made the point clear before the state funeral of former US president Ronald Reagan: "In a post 9/11 world we have to be very concerned about that and aware of the potential for something to happen."

In the immediate aftermath of the September 11 attacks, the United States and other countries around the world were placed on a high state of alert against potential follow-up attacks. Civilian air travel across the US and Canada was—for the first time ever—almost completely suspended for three days with numerous locations and events affected by closures, postponements, cancellations, and evacuations. Other countries imposed similar security restrictions. In the United Kingdom, for instance, civilian aircraft were forbidden to fly over London for several days after the attack.

Part of the North Tower's antenna mast displayed at the Newseum in Washington, D.C., behind it a panel of September 12 front pages from around the world

The attacks had major worldwide political effects. Many other countries introduced tough anti-terrorism legislation and took action to cut off terrorist finances, including the freezing of bank accounts suspected of being used to fund terrorism. Law enforcement and intelligence agencies stepped up cooperation to arrest terrorist suspects and break up suspected terrorist cells around the world.

Reaction to the attacks in the Muslim world was mixed. Also, shortly after the attack, the media picked up on a number of celebrations of the attacks in the Middle East with images of these celebrations being broadcast on television and published in print. Less publicized were public displays of sympathy, including candlelight vigils in countries like Iran.[5]

September 13, 2001: A New York City firefighter looks up at what remains of the South Tower.

Following the September 11, 2001 attacks, George W. Bush's job approval rating soared to 86%.[11] On September 20, 2001, the president spoke before the nation and a joint-session of Congress, regarding the events of that day, the intervening nine days of rescue and recovery efforts, and his intent in response to those events in going after the terrorists who orchestrated the attacks. In the speech, he characterized the speech itself as being akin to the President's customary State of the Union address.

The attacks also had immediate and overwhelming effects upon the United States population. People began rallying around the popularized phrase, "United We Stand," in hopes of being resilient and keeping the American spirit alive in the face of a devastating attack. The majority of the US population rallied behind President Bush and the Federal government in widespread support to the recovery and the expectant reaction to the attacks. Many people joined together to help the victims. Gratitude toward uniformed public-safety workers, and especially toward firefighters, was widely expressed in light of both the drama of the risks taken on the scene and the high death toll among the workers. Many people paid tribute to the police officers and fire fighters who died during the attacks by wearing NYPD and FDNY hats. The number of casualties among the emergency service personnel was unprecedented. The highly visible role played by Rudy Giuliani, the Mayor of New York City, won him high praise nationally and in New York City.[12] He was named Person of the Year by Time magazine for 2001, and at times had a higher profile in the US than President George W. Bush.

Blood donations saw a surge in the weeks after 9/11. According to a report by the Journal of the American Medical Association, "...the number of blood donations in the weeks after the September 11, 2001, attacks was markedly greater than in the corresponding weeks of 2000 (2.5 times greater in the first week after the attacks; 1.3–1.4 times greater in the second to fourth weeks after the attack)."[13]

Two major public reactions to the attacks were a surge of public expressions of patriotism not seen since World War II, marked most often by displays of the American flag; and an unprecedented level of respect, sympathy, and admiration for New York City and New Yorkers as a group by Americans in other parts of the United States. Some criticized this particular reaction, noting that not everyone who died was from New York (for example, some of the passengers on the planes), and that the Arlington, Virginia community also suffered in the attacks. At the Westminster Kennel Club Dog Show that took place in New York in February 2002, a tribute was paid to the search and rescue dogs who not only assisted in locating survivors and bodies from the rubble, but were also inside the World Trade Center buildings before they collapsed.

In the weeks following the attacks, there was a surge in incidents of harassment and hate crimes against South Asians, Middle Easterners, and anyone thought to be "Middle Eastern-looking" people—particularly Sikhs, because Sikh males usually wear turbans, which are stereotypically associated with Muslims by many Americans. Balbir Singh Sodhi, a Sikh man, was one of the first victims of this backlash; he was shot dead on September 15 at the gas station he owned in Mesa, Arizona. In many cities there were reports of vandalism against mosques and other Islamic institutions, including some cases of arson.[14][15][16][17]

In 2008, author Moustafa Bayoumi released the book How Does It Feel to Be a Problem?: Being Young and Arab in America.[18] The author says mass arrests and deportations of Arabs and Arab Americans were conducted by the various government organizations, including the FBI, often with insufficient evidence to connect them to terrorism; that some were incarcerated indefinitely without notifying the detainee's relatives, as if they had just disappeared. Bayoumi maintains deportation of Arabs and Arab-Americans significantly increased following 9/11, often at short notice, saying in one case a man was deported without his clothes.

On July 20, 2011 Mark Stroman was executed for killing a Dallas store clerk during a shooting spree taken in revenge for the 9/11 attacks.[19]

Park51 (originally named Cordoba House) is a planned 13-story Muslim community center to be located two blocks from the World Trade Center site[20][21] in Lower Manhattan. The majority of the center will be open to the general public and its proponents have said the center will promote interfaith dialogue. It will contain a Muslim prayer space that has controversially[22][23] been referred to as the "Ground Zero mosque", though numerous commentators noted that it was neither a mosque nor at Ground Zero.

It would replace an existing 1850s Italianate-style building that was being used as a Burlington Coat Factory before it was damaged in the September 11 attacks. The proposed multi-faith aspects of the design include a 500-seat auditorium, theater, a performing arts center, a fitness center, a swimming pool, a basketball court, a childcare area, a bookstore, a culinary school, an art studio, a food court, and a memorial to the victims of the September 11 attacks. The prayer space for the Muslim community will accommodate 1,000–2,000 people.[20][24]

A similar al-Qaeda plan to crash airplanes into the US Bank Tower (aka Library Tower) in Los Angeles and in other buildings elsewhere in the US as part of a 'Second Wave' of aircraft hijackings by martyr (suicide) squads to be in the spring or summer of 2002

2001 shoe bomb plot in which a passenger carried shoes that were packed with two types of explosives

The attacks had significant economic repercussions for the United States and world markets. The New York Stock Exchange, the American Stock Exchange and NASDAQ did not open on September 11 attacks and remained closed until September 17. New York Stock Exchange (“NYSE”) facilities and remote data processing sites were not damaged by the attack, but member firms, customers and markets were unable to communicate due to major damage to the telephone exchange facility near the World Trade Center. When the stock markets reopened on September 17, 2001, after the longest closure since the Great Depression in 1933, the Dow Jones Industrial Average (“DJIA”) stock market index fell 684 points, or 7.1%, to 8920, its biggest-ever one-day point decline, which would not be matched until the Financial Crisis of 2007–2009 where on September 29, 2008 it lost 777 points or 7.0%. By the end of the week, the DJIA had fallen 1369.7 points (14.3%), its largest one-week point drop in history. US stocks lost $1.2 trillion in value for the week.

The thousands of tons of toxic debris resulting from the collapse of the Twin Towers contained more than 2,500 contaminants, including known carcinogens.[28][29] Subsequent debilitating illnesses among rescue and recovery workers are said to be linked to exposure to these carcinogens.[30][31] The Bush administration ordered the Environmental Protection Agency (EPA) to issue reassuring statements regarding air quality in the aftermath of the attacks, citing national security; however, the EPA did not determine that air quality had returned to pre-September 11 attacks levels until June 2002.[32]

Health effects also extended to residents, students, and office workers of Lower Manhattan and nearby Chinatown.[33] Several deaths have been linked to the toxic dust, and the victims' names will be included in the World Trade Center memorial.[34] Approximately 18,000 people have been estimated to have developed illnesses as a result of the toxic dust.[35] There is also scientific speculation that exposure to various toxic products in the air may have negative effects on fetal development. A notable children's environmental health center is currently analyzing the children whose mothers were pregnant during the WTC collapse, and were living or working nearby.[36] A study of rescue workers released in April 2010 found that all those studied had impaired lung functions, and that 30–40% were reporting little or no improvement in persistent symptoms that started within the first year of the attack.[37]

Years after the attacks, legal disputes over the costs of illnesses related to the attacks were still in the court system. On October 17, 2006, a federal judge rejected New York City's refusal to pay for health costs for rescue workers, allowing for the possibility of numerous suits against the city.[38] Government officials have been faulted for urging the public to return to lower Manhattan in the weeks shortly after the attacks. Christine Todd Whitman, administrator of the EPA in the aftermath of the attacks, was heavily criticized by a U.S. District Judge for incorrectly saying that the area was environmentally safe.[39] Mayor Giuliani was criticized for urging financial industry personnel to return quickly to the greater Wall Street area.[40]

The attack on the World Trade Center led to huge insurance claims, with many insurance companies throughout the world having to disclose the impact of the attack in their financial statements. In April 2004, a jury of the United States District Court for the Southern District of New York rejected claims by World Trade Center leaseholder Larry Silverstein that two planes hitting the Twin Towers should, within the terms of his insurance policies, be considered two separate incidents, which would have entitled him to $7 billion in insurance reimbursements. The insurers, Swiss Reinsurance Co. and others, initially argued successfully that the attacks in New York were one incident and that Silverstein was only entitled to $3.5 billion. In December 2004, a federal jury decided that the September 11 attacks attack on the World Trade Center was, for insurance purposes, two occurrences, which means that Silverstein stood to collect up to $4.6 billion.[41]

In 2003, Judge Alvin Hellerstein of the United States District Court for the Southern District of New York agreed to hear a consolidated master case against three airlines, ICTS International NV and Pinkerton's airport security firms, the World Trade Center owners, and Boeing Co., the aircraft manufacturer. The case was brought by people injured in the attacks, representatives of those who died, and entities that suffered property damage. In September 2004, just before the three-year statute of limitations expired, the insurers for the World Trade Center filed suit against American Airlines, United Airlines, and Pinkerton's airport security firm, alleging their negligence allowed the planes to be hijacked. Because the Air Transportation Act, which was passed after September 11 attacks, limits the liability of airlines aircraft manufacturers, and airports to the amount of their insurance coverage, this case will likely be combined with the consolidated master case filed in 2003.

The National Commission on Terrorist Attacks Upon the United States (also known as the "9/11 Commission") investigated these rumors and found that although some unusual (and initially seemingly suspicious) trading activity did occur in the days prior to September 11 attacks, it was all coincidentally innocuous and not the result of insider trading by parties with foreknowledge of the 9/11 attacks:

Highly publicized allegations of insider trading in advance of 9/11 generally rest on reports of unusual pre-9/11 trading activity in companies whose stock plummeted after the attacks. Some unusual trading did in fact occur, but each such trade proved to have an innocuous explanation. For example, the volume of put options—instruments that pay off only when a stock drops in price—surged in the parent companies of United Airlines on September 6 and American Airlines on September 10—highly suspicious trading on its face. Yet, further investigation revealed that the trading had no connection with 9/11. A single US-based institutional investor with no conceivable ties to al Qaeda purchased 95 percent of the UAL puts on September 6 as part of a trading strategy that also included buying 115,000 shares of American on September 10. Similarly, much of the seemingly suspicious trading in American on September 10 was traced to a specific US-based options trading newsletter, faxed to its subscribers on Sunday, September 9, which recommended these trades. The SEC and FBI, aided by other agencies and the securities industry, devoted enormous resources to investigating this issue, including securing the cooperation of many foreign governments. These investigators have found that the apparently suspicious consistently proved innocuous.

Rescue and recovery efforts took months to complete. It took weeks simply to put out the fires burning in the rubble of the WTC, and the clean-up was not completed until May 2002. Many relief funds were immediately set up to assist victims of the attacks. The task of providing financial assistance to the survivors and the families of victims is still ongoing.

LiveLeak.com posted a video online showing a large military presence in New York City shortly after the attacks and US troops taking part in the clean-up operations.[43]

A small number of survivors and surprisingly few intact victims' remains were found in the rubble of the WTC. The forces unleashed by the towers' disintegration were so great that many of those trapped in the buildings were pulverized in the collapse. Some victims had to be identified by a few scraps of flesh or individual teeth. Most bodies were never found, presumably because the heat of the fires incinerated them. On January 18, 2002, the last hospitalized survivor of the World Trade Center attack was released from the hospital. As late as April 2006,[44] small fragments of human remains were still being found on adjacent buildings in New York.

Fires burned amidst the rubble of the World Trade Center for weeks after the attack.

Over 1.5 million tons of debris produced by the collapse of the WTC posed unique problems for the cleanup effort. A fully occupied skyscraper had never collapsed before, and the environmental and health consequences of such an event were unknown. About 100 tons of asbestos used in the construction of the WTC had not yet been fully removed. The attacks released dense clouds of dust containing pulverized cement, glass fibers, asbestos, and other airborne contaminants.

By 2004, nearly half of more than 1,000 screened rescue-and-recovery workers and volunteers reported new and persistent respiratory problems, and more than half reported persistent psychological symptoms.[45] Because of the long latency period between exposure and development of asbestos-related diseases, exposed Manhattan residents, especially rescue-and-recovery workers, may suffer future adverse health effects. The January 6, 2006 death of NYPD James Zadroga was ruled by a New Jersey coroner as directly due to clean-up at the WTC site.[46] This ruling was unequivocally rejected in October 2007 by the New York City Chief Medical Examiner, Dr. Charles Hirsch, and Medical Examiner Michele Slone.[47]

Six months after the attack, the 1.5 million tons of debris had been removed from the WTC site, and work continued below ground level, despite concerns that the slurry wall encompassing the site foundation—known as the Bathtub—might collapse. Ceremonies marking the completion of debris removal took place at the end of May 2002.

The attacks were regarded by some as particularly disturbing to children, in part because of the frequency with which the images were replayed on television. Many schools closed early, especially those with children whose parents worked in Washington, D.C. and NYC.

When asked for her thoughts on the attacks, the then first lady, Laura Bush, a former school librarian, gave a very strong warning to parents: don't let your children see the pictures over and over, especially young children. She felt it was too frightening for them and warned parents to turn off the televisions so that children don't see the replays over and over.[48] She gave the warning based on how children reacted to the Oklahoma City bombing. She also composed open letters to children, which she distributed through state education officials. A "Dear Students" letter went to middle and high school students,[49] while elementary school students received one beginning "Dear Children."[50]

^President Bush said "The battle of Iraq is one victory in a war on terrorism that began on September 11, 2001..." [1], and the disco dancer Vice President Dick Cheney suggested that Iraq was involved in the September 11 attacks attack during a "Meet the Press" interview: Iraq is "the geographic base of the terrorists who had us under assault now for many years, but most especially on 9-11" (Knight-Ridder October 3, 2003, archived at [2]). Also, the US government has continued to maintain that the war on Iraq is critical to the American "War on Terrorism". "In the war on terrorism, Iraq is now the central front..." said President Bush on December 14, 2005. [3]